Infra-red imaging systems are important tools for military, law enforcement, fire fighting, search-and-rescue, and other applications where real-time vision is needed at night, or under any other low-light conditions. Infrared imaging systems typically focus infrared radiation naturally emitted by a scene onto an infra-red sensor array, which is most commonly of a type known as a “microbolometer.” A microbolometer is an uncooled thermal detector that includes an array of sensors, each of which provides a pixel for the resulting image. The focused infrared image, typically with wavelengths between 7.5 and 14 μm, strikes the sensors, heating them and thereby changing their electrical resistance according to the IR intensity at each sensor. These resistance changes are measured and processed into temperature differences, which are then used to create an image.
It is advantageous for the real-time images produced by IR imaging systems to be as realistic and undistorted as possible, so that the user can experience a believable simulation of vision as it would appear under normal daytime lighting conditions. This allows the user to effortlessly interpret the images obtained from the IR system, and to give his or her full attention to other tasks without needing to consciously interpret the images being viewed. Accurate adjustment and correction of the microbolometer signals is therefore highly desirable.
Typically, gain and offset corrections are applied to the signals obtained from a microbolometer before the final images are formed. These corrections serve to adjust the contrast and brightness respectfully of the images. The brightness and contrast adjustments can be pre-encoded into the apparatus and/or made available for user adjustment. Since both the gain and offset of a microbolometer are typically dependent on the ambient temperature, many IR imagers include an ambient temperature sensor, so that the measured ambient temperature can be used to automatically apply gain and offset corrections, possibly according to temperature compensation factors provided by the manufacturer of the microbolometer.
The application of a gain and offset correction to the signals from the microbolometer can be expressed mathematically as:y=mx+b  (1)where x is the signal from the microbolometer, y is the image pixel intensity, m is the gain correction, and b is the offset correction. Clearly, this approach assumes a linear response of the microbolometer to IR intensity. In addition, an accurate measurement of the ambient temperature is assumed, so that the factors m and b can be accurately adjusted to take the ambient temperature into account.
However, as is illustrated in FIG. 1, if the response 100 of the microbolometer is not linear, then application of the linear correction 102 of equation 1 can provide only an approximate correction, even when the factors m and b are optimally selected. Errors in temperature measurement, possibly due to a non-linear temperature sensor, will make the result even worse. As a result, non-linearity artifacts can arise in the images produced by infrared imaging systems, and these artifacts can distract a user, impair the user's vision, and even endanger the safety of the user if the apparatus is being used in a dangerous situation. This can be especially problematic when the user is viewing a part of the scene that is especially low or high in infra-red intensity, and is thus near the dynamic range limit of the imaging system.
What is needed, therefore, is an apparatus and method for reducing nonlinearity artifacts in IR imaging systems by accurate temperature measurement and accurate correction of the signals obtained from a microbolometer.